This invention relates generally to the field of pumps for use in hydraulic systems, such as power steering systems. In particular, this invention relates to a new design for a variable displacement power steering pump and system.
Variable displacement (VD) power steering pumps utilize hydraulic pressure and pump shaft revolution speed to change the displacement of the pump, thus reducing the input torque requirements on the drive engine""s front end accessory drive (FEAD). The ability of VD power steering pumps to change their displacement in response to pump shaft speed makes the pumps more fuel efficient as a result of reduced input torque requirements. These pumps are commonly used in power steering systems.
With reference to FIG. 1, a typical power steering system is illustrated. The system comprises a steering gear 1, an oil pump 2, a reserve tank 3, hydraulic piping 4, a cooler 5, and, of course, a steering wheel 6. The steering gear 1 is actuated by input from the steering wheel. Oil pump 2 pumps oil through hydraulic piping 4 to the steering gear 1. Oil from the steering gear may be run through cooler 5 after use. Fixed displacement pumps generate excess flow at medium or high speeds, which raises the oil temperature. The heat lost in cooling the oil corresponds to lost power and efficiency. Variable displacement pumps raise the efficiency of power steering systems by reducing the loss of energy caused by surplus flow in fixed displacement pumps. This is accomplished by generating flow that better corresponds to system needs.
With reference to FIG. 2, a variable displacement (VD) power steering pump""s internal rotating group is illustrated. The internal rotating group comprises a rotor 10, vanes 12, cam (or cam ring) 14, pivot pin 16, and outer ring 18, as well as two pressure plates and a wiper seal (not shown). The rotor 10 is connected to a rotating pump shaft 20. The rotor 10 and vanes 12 are surrounded by cam 14, which is coupled to a pivot pin 16 that permits the cam to move its center with respect to the pump shaft center. By altering the relative position of the cam center to the rotor center, the eccentricity of the cam to the pump shaft center can be altered, consequently altering the displacement of the pump. A spring 22 biases the cam towards a predetermined position of maximum eccentricity permitted by the device. For the purposes of this illustration, rotor 10 rotates in the clockwise direction shown by arrow 23.
The general function of such pumps is to hydraulically respond to the needs of the steering system, as well as to changes in engine revolutions per minute (rpm). The pump only provides the amount of flow that is required by the system by varying the displacement of the pump in response to shaft speed and system pressure. As shaft speed increases, the pump output flow increases. When the pump reaches a desired shaft speed, a spool valve and spring combination allows pressure to be exposed on one side of the cam. This pressure causes the cam to move or rotate about pivot pin 16 in the direction shown by arrow 24, decreasing the eccentricity of the cam with respect to the pump shaft center, and thereby decreasing the displacement of the pump. As the shaft speed increases, the pump displacement is decreased in order to provide a steady flow that is metered by an orifice, which is located in the pressure plate. An inlet flow path 26 and an outlet flow path 28 are each indicated by a corresponding numbered arrow in FIG. 2.
Generally, two pressure plates are used to contain the outer ring, cam, rotor, and vanes as a single group. The face details of both plates are mirror images of each other. Each plate serves several functions in the pump. For example, the plates create a seal for the rotating group and provide a path for pump fluid. One of the plates contains a metered orifice for the outlet flow. A pressure port serves as a fluid outlet as the chamber volume decreases in the rotating group. A suction port serves as a fluid inlet to the rotating group as the chamber volume increases. Referring to FIG. 2, the rotation of the shaft is clockwise. If the Figure is reversed, the rotation would be counterclockwise. The under vane ports provide hydraulic pressure behind the vanes to face the vane tips to ride along the cam profile, creating a sealed chamber between the vanes allowing the pump to do work on the fluid.
Further information on variable displacement pumps and power steering systems can be found in numerous patents, articles and books, such as but not limited to U.S. Pat. No. 5,562,432, entitled Variable Displacement Pump Having Throttled Control Passages; Karmel, A. M., xe2x80x9cA study of the Internal Forces in a Variable Displacement Vane Pump, Part 1, Theoretical Analysis,xe2x80x9d Journal of Fluids Engineering, Vol. 108/227, June 1986; and
Mochizuki, Teruhiko, xe2x80x9cDevelopment of the Variable Displacement Vane Pump for the Automotive Power Steering System,xe2x80x9d Report #SAE-930261, SAE, 1993, all of which are incorporated by reference as if reproduced in full herein.
Although the use of variable displacement pumps in place of fixed displacement pumps has increased the efficiency of hydraulic systems, it is desired to further increase the efficiency of variable displacement pumps and the hydraulic systems incorporating them. For example, during straight ahead driving, minimal displacement is necessary, yet often higher displacements are present. This higher pump displacement causes unnecessary input torque requirements on the engine""s front end accessory drive (FEAD). It is desired to reduce such xe2x80x9cparasiticxe2x80x9d losses of power by better matching pump displacement to requirements.
In one embodiment, the present invention provides a variable displacement pump having a fixed displacement mode and a variable displacement mode. The fixed displacement mode is made possible by a mechanical control that can adjust the eccentricity of the cam to provide a fixed displacement. The present invention also incorporates a hydraulic system, comprising a pump drive input, an output driven mechanism, and a variable displacement pump having a fixed displacement mode and a variable displacement mode. In a preferred embodiment, the mechanical control comprises a solenoid-operated connecting rod that directly varies the eccentricity of the cam with respect to the pump shaft center. The connecting rod is operatively connected to the cam to cause the cam to pivot in response to the relative linear motion of the rod. Preferably, in use, the solenoid is selectively activated in response to pump shaft speed and hydraulic pressure, so that, in a power steering system for example, when there is low power steering demand and engine speed is below a predetermined level, the cam is situated for minimum pump displacement.
When a pump constructed in accordance with the present invention is utilized in a vehicle power steering system, the pump will be in fixed displacement mode at low to moderate speeds with no steering input, and will switch to variable displacement mode with steering input or at higher speeds.
The present invention also incorporates a method for improving the efficiency of variable displacement pumps and systems using them by providing a fixed reduced displacement mode. In an embodiment, the method comprises forcing the cam of a variable displacement pump to a position that reduces displacement in response to reduced requirements from the output driven mechanism, e.g., power steering gear, or low or moderate pump shaft speed.